avcodec/alsdec: Limit maximum channels to 512

There seems to be no limit in the specification and upto 64k could be stored
512 is choosen as limit as thats the maximum in a conformance sample

An alternative to this patch would be a max_channels variable

Fixes: OOM
Fixes: 16200/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ALS_fuzzer-5764788793114624

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSuggested-by: Thilo Borgmann <thilo.borgmann@mail.de>
Reviewed-by: Thilo Borgmann <thilo.borgmann@mail.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/alsdec.c

@@ -348,6 +348,11 @@ static av_cold int read_specific_config(ALSDecContext *ctx)
     if (als_id != MKBETAG('A','L','S','\0'))
         return AVERROR_INVALIDDATA;
 
+    if (avctx->channels > FF_SANE_NB_CHANNELS) {
+        avpriv_request_sample(avctx, "Huge number of channels\n");
+        return AVERROR_PATCHWELCOME;
+    }
+
     ctx->cur_frame_length = sconf->frame_length;
 
     // read channel config